1. Field of the Invention
The invention relates to a gas-dynamic pressure-wave supercharger for the supercharging of an internal combustion engine with an exhaust blow-off valve, which pressure-wave supercharger has a rotor housing with a cell rotor, in which the exhaust gas of the internal combustion engine compresses the combustion air required by the internal combustion engine, furthermore with an air housing, through which atmospheric air is taken in and, after compression in the cell rotor, is fed as charge air to the internal combustion engine, as well as with a gas housing, via which the exhaust gas coming from the internal combustion engine is directed into the cell rotor and, after its expansion in the cell rotor, is directed away via an exhaust outlet connection into an exhaust manifold, an exhaust bypass in the gas housing, with a medium-controlled gate, connecting the high-pressure gas inflow duct to the low-pressure gas outflow duct, which gate i$ in effective connection with a control device actuated by a process pressure of the pressure-wave supercharger.
The use of an exhaust bypass in the case of small engines for passenger cars supercharged by means of pressure-wave machines--with which the peak pressure is limited and which have a broad speed range available --may well be viable. Since such engines have a flexible torque, by virtue of the flat pressure characteristic over the complete engine speed range, here however--in comparison with exhaust turbo charging--on the one hand less exhaust gas has to be blown off into the exhaust and on the other hand blowing off does not have to take place until higher engine speeds. Consequently, the poorer specific fuel consumption due to the unutilized blowing-off only occurs in a narrow range which, experience shows, occurs rarely in the case of a passenger car.
2. Description of Background
A controlling of the charge air pressure by selective blowing-off with a pressure-wave machine mentioned at the beginning is known from British patent specification 775,271. If the exhaust-gas pressure exceeds a pre-determined value, a spring-loaded gate arranged in a bypass between high-pressure gas inflow duct and low-pressure gas outflow duct opens. A part of the exhaust gases passes through this bypass directly into the exhaust without going through the pressure-wave process. With such an arrangement, however, the blown-off exhaust gases flow with a speed component transversely to the flow direction of the exhaust gases into the exhaust outlet connection, resulting in the disadvantages described below.
For a satisfactory effective function of the pressure-wave supercharger, the expanded exhaust gases, once they have done their compression work, must be scavenged together with the mixture of air and exhaust gas which has formed in the mixing zone, i.e. in the region of the separating surface of air and exhaust gas, completely into the exhaust outlet connection. This scavenging is supported by the intake air, which enters into the rotor cell on the side opposite the exhaust openings and, as a result, the rotor is cooled at the same time. In order to achieve satisfactory compression efficiencies, however, a further cooling of the rotor is necessary. For this purpose, the pressure-wave supercharger must take in more air than it gives off compressed air to the engine. This air additionally taken in is called scavenging air and the ratio of scavenging air stream to charge air stream is called the "degree of scavenging" of the pressure-wave supercharger. This degree of scavenging drops with increasing engine speed and decreasing engine loading.
As in the case of a turbo charger, with a pressure-wave supercharger, the blowing-out through the waste gate primarily impairs the overall efficiency, and consequently the specific fuel consumption, but not the degree of scavenging. This is because the scavenging energy reduces approximately proportionally to the compression energy.
With small blow-off streams, the transverse component of the flow into the exhaust duct does not represent a serious impairment of the exhaust stream and consequently of the degree of scavenging. With greater blow-out streams, however, the scavenging is appreciably worsened by the greater transverse component of the entry speed and consequently the compression efficiency is also impaired.
In addition, full-load operating points at high speeds are characterized by an inadequate low-pressure scavenging. The cause resides in the poor distribution of the energy still present in the rotor cells along the low-pressure opening. The speed profile has two pronounced outflow fields, namely one field with high outflow speed in the region of the low-pressure opening edge and one field with low outflow speed in the region of the low-pressure closing edge. This profile is predetermined by the pressure-wave process.